Figure 1. Proteolytic Capacity Increases During Transient Adaptation to H₂O₂.

MEF cells were grown to 50% confluence and exposed, in PBS, to an adaptive pre-treatment of 2 μmol H₂0₂ per 10⁷ cells. Successful transient adaptation (peaking at about 24 hours and then declining) was confirmed by increased capacity to survive a subsequent (much higher) challenge dose of H₂O₂ that, without adaptation, significantly decreased cell proliferation and DNA replication, and significantly increased accumulation of oxidized proteins (confirmatory data not shown at this point, but given as part of Figure. 7) as previously described [25, 26]. At various time points after exposure, cells were harvested and lysed then suspended in 50mM Tris, 25mM KCl, 10mM NaCl, 1mM MgCl₂, (pH 7.5). Proteolytic activity assays for degradation of either [³H]Hb_ox [5, 15] or suc-LLVY-AMC [48, 49] were performed as described in Materials & Methods. Values are means ± SE, n = 3. The experiment was repeated in MEF cells grown to only 20% confluence and adapted by pre-treatment with 250 nmol of H₂0₂ per 10⁷ cells, with very similar results (data not shown.)